In retinal rods, incident light triggers a highly amplified enzymatic cascade that results in the closure of cyclic GMP-activated channels in the plasma membrane. The gain of the phototransduction cascade is set by the ambient light level. In darkness, a retinal rod can detect a single photon of light, but its dynamic range is limited. The presence of background light, however, desensitizes the rod, shifting and extending its dynamic range. This process, known as light adaptation, maximizes the rod's ability to detect contrast over a broad range of light intensities. In this application we propose to investigate feedback mechanisms that may contribute to light adaptation by decreasing the efficiency of coupling between the light stimulus and the cellular response. Although many steps of the tightly regulated phototransduction cascade are potential targets for the feedback mechanisms responsible for light adaptation, this proposal focuses on two recently discovered mechanisms which seem particularly well-suited to play a role in this process. First, the inhibitory subunit of the cGMP-specific phosphodiesterase (PDEgamma) is phosphorylated in an activity-dependent manner; this phosphorylation may block activation of PDE catalytic activity by transducin, the G- protein of retinal rods. Second, the affinity of the cGMP-activated channels for cGMP is regulated by the binding of transition metal divalent cations and the calcium-calmodulin complex. These mechanisms may either strengthen or weaken the coupling between cGMP hydrolysis and channel closure. The molecular mechanisms responsible for both of these processes and their contribution to light adaptation will be studied using a combination of biochemical, molecular biological, and electrophysiological methods. These experiments will increase our understanding of how retinal rods generate a neural signal in response to incident photons, and how rods can modify their behavior in response to changing stimuli. A deeper understanding of retinal rod function will provide a foundation for the treatment of diseases like retinitis pigmentosa and night blindness. Furthermore, because retinal rods utilize common signaling mechanisms, this research should also provide insight into the regulation of signal transduction pathways throughout the body.